1. Field of the Invention
The present invention relates to a rate control system of a TCP layer in communications conducted between two terminals which are connected via a wireless link.
2. Description of the Related Art
A TCP (Transmission Control Protocol) corresponds to a transport layer, which is the fourth layer of an OSI (Open System Interconnection) and is a transport layer protocol, and is currently installed in many personal computers and workstations because the TCP guarantees the high-reliability of end-to-end transmission and is suited for data communications.
The TCP prevents network congestion from occurring by rate control using a slow-start function (function to start transmission at a low transmission rate and to gradually increase the transmission rate) and implements high-reliability compared with re-transmission control and window control. In particular, the slow-start function is an algorithm for implementing recovery from the congested state causig a packet discard by greatly reducing the transmission rate once and then gradually increasing the transmission rate, and is a excellent feature of the rate control in the TCP. However, if this technology is applied to a wireless environment without modification, inefficient operation occurs due to this slow-start function and a reduction of throughput occurs when there is a packet discard. Therefore, improvement is desired.
The slow-start function, which is a feature of the rate control of the TCP is a system for reducing the transmission rate when a packet is discarded and for gradually increasing the rate based on an acknowledgement (ACK) from the opposite terminal, which is described with reference to FIG. 1.
FIG. 1 shows an example of the rate control of a conventional TCP. The horizontal and vertical axes in FIG. 1 indicate time and transmittable data size (WS: Window size), respectively. The dotted lines represented by a in the upper section of FIG. 1 indicate the change of control using a window (data size receivable at the opposite terminal), and a plurality of vertical solid lines b1-bn indicate the data size transmitted by a sender. Circles separating the vertical solid lines indicate the maximum segment size of the TCP, and data transmitted from the TCP are divided into a size that is not more than this maximum size and are transmitted. Since there are no data in a receiving window when transmission is started, data equivalent to the window size can be originally transmitted at one time. However, in order to avoid the congestion of a network due to the rapid increase of the rate, the data size to be transmitted at one time is restricted by the slow-start.
Specifically, first the data size represented by b1 is transmitted, and when an ACK is received from the opposite terminal, data are transmitted at a higher transmission rate, such as b2, b3, . . . (see section A in FIG. 1). In this way, the transmission rate continues to be increased. However, the more ACKs that are received, the looser the slow-start restriction becomes. The rate control gradually shifts to window control (see section B in FIG. 1). In this way, if there is a packet discard while transmission continues at a transmission rate bn, an ACK is not returned from the opposite terminal. Therefore, presently a re-transmission time-out (represented by RTO) occurs. If the re-transmission of data is started after the re-transmission time-out occurs, the transmission rate begins with data size b1 and then is gradually controlled by slow-start control (see section C in FIG. 1).
In the meantime, a communication form in which a laptop personal computer, etc., is connected via a modem to a portable terminal explosively spread as an audio communications tool, such as cellular phones, PHS (Personal Handy-phone System), etc., and data communications (TCP communications) conducted over a wireless link, is also being spread.
However, the TCP does not originally take into consideration that a wireless link is applied to a physical layer, and it is a protocol which is designed on the assumption that a physical line is a wired line. Specifically, the above-described slow-start function is based on a concept of congestion avoidance, and the dissolution of congestion is intended by reducing the transmission rate. In other words, the TCP is an algorithm which is introduced based on the assumption that the cause of a packet discard is congestion.
However, in the case of TCP communications via a wireless link, a packet discard can occur not only due to congestion, but also due to a wireless characteristic, such as fading, etc. Although in such a case, the slow-start function works in a TCP layer and no effect can be obtained, since the increase/decrease of the transmission rate and an error characteristic in a wireless link are independent phenomena. Since there is a packet discard in the conventional rate control system of the TCP layer, the transmission rate is reduced although no effect can be expected, throughput is reduced without benefit, which is a problem.
Therefore, an object of the present invention is to provide the rate control system of a TCP layer suited for a wireless environment where a fruitless reduction of throughput can be avoided even in communications via a wireless link.
According to the present invention, if the discard of a packet transmitted via a wireless link is detected in a prescribed layer, in order to solve the above-described problem, a layer with a slow-start function is notified of the packet discard.
Then, the slow-start function can be operated according to the peculiar characteristic of a wireless link, even if the slow-start function assumes a wired link and the protocol does not assume a wireless link, and thereby a fruitless reduction of throughput can be prevented in communications via a wireless link.
According to one aspect of the present invention, if the discard of a packet transmitted via a wireless link is detected, the packet is re-transmitted without the slow-start performance.
In this way, if a packet discard occurs due to a cause other than congestion, the packet can be re-transmitted without the reduction of the transmission rate, and as a result, the occurrence of the fruitless reduction of throughput can be prevented.